In this era of social media, where we are all super-connected, the most ingenious do-good movements can begin with a tiny idea.

Make ’em viral and they will spread.

It happened with “The Last Text,” the riveting video that AT&T (ATT) produced to coax people like you and me not to text and drive. The 10-minute film, which I wrote about on Postcards last month, has drawn more than two million views. And no doubt, it has saved lives.

I love the notion of building a big movement from a small idea. I discovered another example last evening, at a dinner hosted by Desiree Gruber, whose stock and trade is building big from small. (Her New York-based marketing firm, Full Picture, came up with the Victoria’s Secret Fashion Show to buff that client’s brand image and created Project Runway to elevate another client, model Heidi Klum, who co-produces the TV hit with Gruber.) Last night Gruber was in full philanthropic mode, “marketing” Caryl Stern, the U.S. CEO of UNICEF, whom I sat across from at dinner. UNICEF has built many programs from simple ideas to save young lives. This week: During World Water Week, UNICEF is urging restaurant patrons to donate a dollar for each glass of tap water that typically comes free.

The nationwide campaign is called the Tap Project. (“When you take water, give water.”) Ever since an ad man named David Droga of Droga5 dreamed it up for UNICEF in 2007, it has helped raise $2.5 million to supply safe drinking water to children around the world.

This year’s twist taps celebrities like Taylor Swift, Robin Williams, Selena Gomez and Rihanna. They’ve bottled tap water at home and poured it into fancy bottles. The “Celebrity Tap Packs” help raise more funds for the most needy children worldwide.

And how great is the need? Of the 22,000 children who die every day, 4,100 would not if they had safe water, Stern told us.

This past week, Stern has had some very late nights, trying to help Japan, where contaminated water is just one of a million needs. Japan happens to be the world’s No. 1 donor to UNICEF, Stern told me. Despite that generosity, UNICEF’s efforts to persuade people to give to the Japan relief fund has been very difficult. “There’s an impression that Japan doesn’t need our dollars,” she explained, adding that she’s spending a lot of time assuring skeptics that UNICEF is not taking money from children in Africa to aid children in Japan.

All the money that UNICEF takes in for Japan will go to Japan, Stern says. Though $2.5 million to date pales to the donations that flowed in last year to help victims of the earthquake in Haiti.

What to do? Well, you can help. You can give to UNICEF’s Japan fund.

Or get creative. Think up a campaign that could help bring in donations for thousands of Japanese kids who have, in an epic moment, become orphans — and for many more kids who lost their homes and their schools. You can send your idea by posting a comment below.

Remember, a big movement begins with a small idea. Make it viral.